What's boosting protons to energies near 1,000 times those we can reach at the LHC?

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We're rightly proud of the Large Hadron Collider, which accelerates protons up to 7 Tera-electron Volts before smashing them together. But the Milky Way regularly hurls protons toward Earth that have energies a thousand times higher than that, in the Peta-electron Volt (PeV) range. Astrophysicists refer to the mysterious sources of PeV particles as “PeVatrons."

What can possibly be boosting particles to these levels? A new paper figures out how many of the high-energy protons are being produced and finds one plausible source: the supermassive black hole at our galaxy's center.

Rather than looking for high-energy protons, the authors track them indirectly. Depending on their interactions with the environment, the accelerated protons can produce gamma rays, which are high-energy photons. The energy of these photons are related to the energy of the initial particles.

To observe the gamma rays, the authors rely on the High Energy Stereoscopic System, or HESS. This is a series of telescopes that an international collaboration has built in Namibia. HESS can track gamma rays that range from GeV to TeV in energy, allowing the spectrum of the sources to be determined.

HESS and other observatories have imaged various sources of gamma rays in the past, but all of them had a cutoff. Below a specific energy, gamma rays were common; above it, they became rare. The cutoff energy puts a limit on the energy of the protons that created the gamma rays—they couldn't have been that energetic or the cutoff would be much higher. None of the sources the authors looked at could be the mysterious PeVatron.

The new work with the HESS telescope involved a careful imaging of our galaxy's core. The telescope was able to spot gamma rays that were higher energy than these earlier cutoffs, indicating there were protons of the right energy about. And the data matched a specific type of gamma ray production, where the high energy one interacts with a hydrogen atom's proton in one of our galaxy's gas clouds. That interaction produces a particle called a pion, which decays in a way that releases a gamma ray.

With all that understood, the authors were able to work backward. Based on the number of gamma rays they detected, they could figure out how many protons must be accelerated. From there, it was possible to estimate the total amount of energy involved (1049 ergs, or about (1026 megatons), a figure the authors call "rather modest." It's possible for the shockwave of a single supernova to accelerate sufficient particles to these energies to match this flux.

But the authors don't think that's what's happening. The shockwave of a supernova only has sufficient energy for about 100 years. To keep a constant supply of high-energy protons, you'd need a steady pace of supernovae in the galactic core, a rate that the authors find implausible.

Instead, they suspect the obvious source at the galactic core: the Milky Way's very own supermassive black hole, called Sgr A*. Right now, it's not outputting enough energy to be a PeVatron; it falls short by a factor of about a thousand. But all it takes is a stray bit of matter falling in to boost the power of Sgr A* considerably, and the authors suspect this has happened often enough in the past to keep high-energy particles flowing.